Four-wire transmission circuit thermistor control



Feb. 14, 1950 E. H, LANHAM 2,497,779

FOUR-WIRE TRANSMISSION CIRCUIT THERMISTOR CONTROL Filed ook. 9, 194e h/ TR. i

LLS.

/NVE/vof? E. H.' LANHAM MCMO a r rom/EV fatented 1.4,

' UNITEDr NSTATES PATENT OFFICE FOUR-WIRE TRANSMISSION CIRGUT 1 THERMISTOR CONTROL `Edward. H. Lanham, Montreal, Quebec, Canada', assignorhby mesne assignments, to Northern Electric Company, Limited, Montreal, Quebec,

y Canada, a corporation of Canada Application October 9, 1946, Serial No. 702,110-

This invention relatesto four-wire transmission systems and more particularly to arrangements for controlling transmission in signaling transmission systems and preventing singing in transmission systems in which separate two-wire paths are provided for transmissionin opposite directions.

The lprincipal proved operation in singing prevention circuits by means of inexpensive apparatus..n y

Arrangements for preventing `singing in fourwire circuits are numerous `and wel1 known in the art. Echo Suppressors for instance, of which there are many include the singing prevention function. There arepatleast two basically different types of echo Suppressors and singing prevention circuits, namely voice operatedmagnetic relays which actuate relay contacts `to open and close one of the two-wire `paths in response to transmission in the other and variable impedance shunting elements in which thevimpedance of an element shunting one two-wire branch is varied in response to transmission in the other.

Among these different types of circuits arel circuits of considerable elaboration which are entirely effective but relatively expensive and others which are simple andminexpensive but which are not entirely satisfactory. As is generally understood in the art'manyelaborate voice operated relay switching arrangements, while satisfactory in certain applications, such as` in long toll lines and in long distance radio tele-H phone circuits in whichthe initial cost of expen-V sive relay switching circuits Vand large annual maintenance charges can be justified, are unsuitable in otherapplications where expense is animportant factor. For instance in a so-called local loud-speaker telephone installation, such as may be employed in interconnecting two oihcesin the same building and which is subject to singing, the high cost of elaborate circuits would be prohibitive. Attempts have been made to devise inexpensive anti-singing ,circuits but so far as applicant is aware none of these has been entirely satisfactory. For instance, it has'been found that certain variable impedance shunting arrangements which appeared promising in that they eliminated magnetic relays, which are relatively expensive and which require frequent visits of maintenance men to keep them in adjustment, do not satisfactorily eliminate singing.

A really effective shunt across one two-wire path, the impedance of which shunt can be controlled in response to transmission in the other two-wire path. appears well suited for employobject vof'my invention is im-v 5 Claims. (Cl. 179-4) ment as an anti-singing device in four-wire loudspeaking telephone circuits. Such an larrangement would not require visits of a telephone maintenance man to the customers premises as would an arrangement employing magnetic re-` lays, the contacts of which require periodic ad@ justment.

posed but reach has been subject to some difficulty which prevented its satisfactory utilization. For instance, in one arrangement a coil wound on a permalloy core` has been employed as the shunting element for one two-wire path. rIhe magnetization of the core and the impedance of the shunting. coil have been controlled in response to transmission in the opposite branch. It has been found, however, that while the contrdl branch is transmitting, during which interval the impedance of the shunting coil should remain at a low fixed value, the impedance 4is not constant, but` varies considerably and the varia-4 tion gives rise to singing in the system. The present invention corrects this diiculty by providing a shunt which remains at a constant lowr value while the opposite two-wire path is transmitting.

The invention may be understood from the following description when read with reference to the associated drawing, showing a preferred embodiment of the invention, as well as from refer--v ence to the claims herein.

Refer now to the drawing which shows a transmitter I, which may for instance be a wellknown microphone transmitter, connected to conductors 2 and 3 of a iirst or transmitting two` wire path which extends through transmitting amplifier 4 to the primary winding 5 of trans# mitting transformer t.` The secondary winding 'l of the transmitting transformer is intercon' nected through conductor 8 and the primary winding 9 of receiving transformer I0, to the two line conductors II and I2. The secondary I3 of receiving transformer I0 is connected through receiving amplifier I4, and the primary and sec` yondary windings I5 and I6 of transformer I1, conductors I8 and I9 of the second, or receiving, two-wire path, and through primary and secondary windings 20 and 2I of transformer 22 to the loudspeaker receiver 23. The inner terminals of the split windings I6 and 20 are each interconnected through condensers 24 and 4U. Bridged across the transmitting conductors ,2, 3 is the primary winding 25 of transformer 26. The right-hand terminal of secondary winding 21 of transformer 26 is connected to the control A number of relatively inexpensive shunting arrangements have heretofore been proelement or grid and the left-hand terminal of winding 2`I is connected to the grounded cathode of tube 2B. Vacuum tube 28 should have a relatively high output and to this end may for instance be a beam power tube such as the R. C. A. 6L6 tube. v The grid is biased with lrespect to the cathode by battery 29, the positive` terminal of which is connected to the grounded cathode and the negative terminal of which is connected through resistance 3U to the control grid. The output circuit of tube 28 may be traced from grounded positive battery 3I through thermistor 32, sliding contact 33 and variable resistance 34, conductor I8 and conductor 35 to the anode of tube 28. Bridged between the anode and cathode of tube 28 is a circuit consisting of resistance 36 and condenser 31. The screen grid of tube 28 is connected to the junction between resistance 3B and condenser 31. The cathode oi tube 28 is indirectly heated by a filament which may be connected to a source of battery not shown. The tube is equipped with secondary beam directing cathode plates which tend to direct and to concentrate the electron stream and to prevent secondary emission. The outer terminals of the variable impedance thermistor element 32 and its associated variable resistance 34 are connected between conductors I8 and |59.

The operation of the circuit 4is as follows. When signals are impressed on the transmitter I, the corresponding variations in voltage .are impressed through amplifier 4 and transformer 6 through conductor 8 and the secondary 9 of transformer I0 onconductors Il and I2 which extend to the connected station. The variations in voitage are simultaneously impressed through transformer 2S on the input circuit of tube 28. Normally tube 28 is lbiased to cut-oil` 'by negative battery 29, so that no current flows through thermistor 32. The characteristics of thermistor 32 are such that when no current flows through it its impedance is a maximum. When the impedance of thermistor 32 is a maximum the shunt is ineffective and the receiving branch is therefore in a condition to receive rsignals incoming over conductors Il', I2 through the two-wire -receiving branch I8-I9. The input circuit of tube 28 is so adjusted that its l'bias is 'overcome by each positive half cycle of 'the voltage wave in conducto'rs 2, 3. Thisresults, because of the characteristics of the selected type yof tube, in 4a relative- 1y heavy current flowing in the output circuit through thermistor 32 during each positive half cycle. The tube is so'chosen that the vmagnitude of its output current is sufficiently large that the impedance o'f thermistor 32 is lowered in its minimum value and remains at its minimumvalue as long as transmission continues inthe transmitting branch. During this interval the input to the loudspeaker is effectively short-circuited. The characteristics of the thermistor selected should be such that for the output current of the tube there is suflicient lag in its response so that its impedance remains at a low Value during'the intervals between successive positive half cycles. The output of the tube is preferablyrelatively high topromote this effect by delaying the cooling of the thermistors between successive positive half cycles.

Incorporated herein by reference are'the following patents disclosing thermally sensitive-resistors or thermistors as they are called Vand-'the design of a thermistor having the required characteristics maybe in accordance with the disclosures therein: G. L. Pearson 2,'1842187/December 26, 1939; G. L. Pearson et a1. 2,253,577, August 26, 1941; G. L. Pearson 2,280,257, April 21, 1942; and G. L. Pearson 2,276,864, March 17, 1942.

It has been found `that through a proper choice of tube and thermistor, it is a relatively simple matter to maintain a shunt of suciently low impedance so as to eifectively block input into the receiver 23 throughout the interval during which transmission continues in the transmitting branch. The variable resistance 34 provides an adjustment in the shunt branch to care for variations in the elements and in local conditions.

By means of the above described arrangement it is possible to eliminate the manually operable singing prevention switching device which is ordinarily 'employed to alternately enable and disable the transmitting and receiving two-wire branches for opposite directions of transmission during a conversation over the circuit. Such a switching device has obvious disadvantages, chief among which vare caused by occasional failure to manipulate the switch with resultant failure to transmit. rIvhe switching herein is eiiected automatically by means of a few inexpensive elements which effectively prevent singing.

What is claimed is:

1. A four-wire transmission circuit having a first and a second two-wire transmitting path, a control branch interconnecting said paths, said control branch having a thermistor connected in shunt across said first path, a vacuum tube in said branch, an input circuit Iof said tube connected to said second path, an output circuit for said tube including said thermistor, andmeans .for varying the current through said thermistor lin response to transmission in said second path to vary the impedance of said shunt to reduce the effect of acoustic coupling between said paths.

2. A four-wire loudspeaker telephone transmission system having a iirst vand a second two-Wire transmission path, a control branch interconnect ing said paths, said branch having a thermistor connected in a shunt across said first path, a vacuum tube in said branch, an input circuit for said tube connected to said second path, an output circuit for said tube `including `said thermistor and means responsive to the positive half cycles of transmission voltages in said second path vfor generating Aa current of relatively large magnitude in said output circuit and conducting said current through said thermistor to maintain'the impedance of said shunt atv a minimum during both positive and negative half cycles of transmission voltages while transmission continues in said second path. Y

3. In an electrical communication circuit, va first path in which controlling voltages are generated, la second path Ato tbe controlled by said voltages, a bridge connecting said paths, a vacuum tube in saidbridge, a thermistor=in said bridge, the input of said tubeconnectedtosaid first path, the voutput vof saidtube connected'tosaid thermistor, said thermistor connected to said second path, a biasing circuit for said'tube,'said tube 'biased'so as to be responsive only to positive 'half cycles of said voltage, said tube having-an output'current of sufficient magnitude to maintain said thermistor at low impedance during ythe intervals between Vsaid positive half cycles of said voltage.

4. In a four-wire loudspeakingtelephone system, a transmitting'branch comprising a transmitter connected to a two-wire transmitting branch, a vloudspeaker receiver connected 'to la two-wire receiving'braxch, said branches coupled through individual transformers to a singlewtvroway, two-Wire line, a, thermistor shuntedjacioss said two-wire receiving branch, a transmission control path interconnecting said transmitting branch and said receiving branch, saidr-'fcontrol path including a vacuum tube having an input circuit connecting to said transmitting` branch and an output circuit extending through said thermistor, said tube biased so that it is normally cut oi and is responsive to positive half fcycles of voltage due to voice current in said transmitting branch, said tube having an outpui'l current of relatively large magnitude, so as to heat said thermistor sulciently in response to said positive half cycles so as to maintain the impdance of said thermistor at a relatively low valve between said positive half cycles, so ras to prevent transmission in said receiving branch whenjransmission is proceeding through said transmission branch.

5. In a four-wire loudspeaker telephone system, individual one-way, two-wire transmitting and receiving branches coupled to a single' two-Way two-wire circuit, a telephone transmitter and a loudspeaker telephone receiver connected individually to said transmitting and receiving branches respectively, a thermistor shunting said receiving branch, a control path interconnecting said transmitting and receiving branches, said control path having a vacuum tube of the beam power type, said tube having an input connected to said transmitting branch and an output path extending in series through said thermistor, said tube having a biasing circuit and normally biased to cut off, said tube responsive to positive half cycles of voltage due to voice currents in said transmitting branch, said tube having an output current of sufficient magnitude to maintain said thermistor at a suiciently low impedance to prevent transmission n said receiving branch while said transmitting branch is in the transmitting condition.

EDWARD H. LANHAM.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date` 1,491,340 Curtis Apr. 22, 1924 2,205,142 Hoard June 18, 1940 2,282,405 Herri-ck May 12, 1942 2,287,998 Johnson June 30, 1942 2,360,940 Edwards Oct. 24, 1944 2,279,768 Van Wynen July 3, 1945 

